1. Field of the Invention
This invention relates to the field of data communication, and in particular to a method of reducing signal peaks in a Discrete Multitone (DMT) signal.
2. Description of Related Art
DMT or Discrete Multitone is a multicarrier transmission technique that uses a Fast Fourier Transform (FFT) and inverse FFT to allocate transmitted bits among many narrow narrowband QAM modulated tones depending on the transport capacity of each tone. This of course can vary with transmission conditions. As is known in the art, QAM (Quadrature Amplitude Modulation) is a passband modulation technique wherein information is represented as changes in carrier phase and amplitude.
DSL or Digital Subscriber Line is a system wherein a non-loaded local loop provides a copper connection between a network service provider and customer premises. DMT is a common form of modulation used in DSL systems. In a DMT based DSL system, the required peak-to average ration (PAR) of a signal is 15 dB for the probability of a clipping occurring to be 10−8 (assuming a Gaussian distribution).
A large PAR value will seriously reduce the signal dynamic range. On the one hand, any peak value will cause signal saturation and the error will spread at all frequency subcarriers. In the worst case, the entire frame of a signal can be wiped out. On the other hand, if the PAR is increased so that the signal has less chance of being clipped, the dynamic range is lost. For the case where PAR=15 dB, the signal will normally be transmitted 15 dB below its peak level.
In a DMT system, multiple QAM constellations are modulated on different carrier frequencies. In the time domain, the signal has variable levels. Normally, the maximum peak-to-average ratio ranges from 27 dB to 39 dB depending on the size of FFT. To increase the signal dynamic range and reduce the PAR, several methods have been used in DMT based DSL systems. The most efficient method is to use a special waveform known as a signature waveform. This is a time domain signal which has a large peak in it and is otherwise small at other time instants. Whenever the signal is larger than a maximum level, the signature waveform is subtracted from the signal so that the signal will not be saturated. However, addition of the signature waveform will generally cause distortion to the transmission signal.
Prior art peak reduction systems are described, for example, in J. Tellado and J. Cioffi, “PAR Reduction in Multicarrier Transmission System”, ANSI Contribution T1E1.4/97-367, Sacramento, Calif., December 1997; and A. Gatherer and M. Polley, “Controlling Clipping Probability in DMT Transmission”, 1997 Asilomar Conference, November, 1997, the contents of which are herein incorporated by reference.
An object of the invention is to alleviate this problem.